Pressure Effect on the Solvation of Tetraalkylammonium Ions in Water.

Abstract

In order to elucidate the pressure effect on the hydrophobic hydration shell, the limiting molar conductivities of tetraalkylammonium ions (Me4N+ to Bu4N+) in H2O and D2O were determined at 5°C as a function of pressure up to 196. 1 MPa. The rotational correlation times of D2O molecules in pure D2O (τc0) and coordinated to Me4N+ and Bu4N+ ions (τc) were also estimated at 6. 9°C up to 294. 2 MPa from 2H NMR spin-lattice relaxation times (T1). The residual friction coefficient (Δζobs) obtained from the limiting molar conductivity of the ion (λ0) became larger with increasing ionic radius and pressure except for Pr4N+ and Bu4N+ ions in D2O at lower pressure. These results suggest that the structure of the hydrophobic hydration shell is not weakened by pressure as much as that of bulk water, which is in qualitative agreement with the pressure dependence of τc (Bu4N+) /τc0. However, a larger value of τc (Me4N+) than that of τc0 does not always correspond to a negative value of Δζobs (Me4N+) indicating a structure breaking property of this ion.